1. Field of the Invention
The present invention relates to a differential amplifier circuit, an operational amplifier including the differential amplifier circuit, and a voltage regulator circuit.
2. Description of the Related Art
A differential amplifier circuit and a low-dropout regulator comprising such a differential amplifier are disclosed in, for example, Japanese Patent Application Publication No. 2009-053783, No. 2007-249712, No. 2002-157031, and Japanese Patent No. 4236586.
FIG. 4 shows an example of a prior art differential amplifier circuit. In general, a differential amplifier circuit is configured to heighten input impedance as much as possible or increase an output impedance of an input signal source as much as possible. However, an extremely high output impedance of the input signal source may affect input signals. For example, a fluctuation in a voltage of a power supply source VDD is transmitted in an almost same magnitude to a node A connecting transistors M3, M4 constituting a constant current source and transistors M1, M2 constituting a differential input stage, and inputted thereto as input signals VIN1, VIN2 via a gate to drain capacitance of the transistors M1, M2. With provision of a bypass condenser bypassing the node A of the transistors M1, M2, only a voltage difference input via the gate to drain capacitance is transmitted to the node A, reducing an influence from the voltage fluctuation in the power supply source Vdd effectively. However, there is a drawback that a size of an LSI is increased by addition of the bypass condenser.